Document and Entity Information - shares	3 Months Ended
	Sep. 30, 2023	Nov. 13, 2023
Document and Entity Information [Abstract]
Document Type	10-Q
Document Period End Date	Sep. 30, 2023
Document Transition Report	false
Entity File Number	000-56269
Entity Registrant Name	MARATHON BANCORP, INC.
Entity Incorporation, State or Country Code	MD
Entity Tax Identification Number	86-2191258
Entity Address, Address Line One	500 Scott Street
Entity Address, City or Town	Wausau
Entity Address State Or Province	WI
Entity Address, Postal Zip Code	54403
City Area Code	715
Local Phone Number	845-7331
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		2,157,497
Entity Central Index Key	0001835385
Current Fiscal Year End Date	--06-30
Document Fiscal Year Focus	2024
Document Fiscal Period Focus	Q1
Amendment Flag	false
Document Quarterly Report	true